Check -mtune even when --with-tuning is used

configure.ac

@@ -1202,20 +1202,23 @@ else
     elif test "${target_cpu}" = "powerpc"; then
         CFLAGS_TUNING="-mtune=750";
     fi
+fi
 
-    dnl NOTE: this can't be cached cleanly
-    AS_IF([test "${CFLAGS_TUNING}"],
-       [CFLAGS_save="${CFLAGS}"
-        CFLAGS="${CFLAGS} ${CFLAGS_TUNING}"
+dnl NOTE: this can't be cached cleanly
+AS_IF([test "${CFLAGS_TUNING}"],
+   [CFLAGS_save="${CFLAGS}"
+    CFLAGS="${CFLAGS} ${CFLAGS_TUNING}"
 
-        AC_MSG_CHECKING([whether $CC accepts ${CFLAGS_TUNING}])
-        AC_COMPILE_IFELSE([ ], [tuning="yes"],
-                               [CFLAGS_TUNING=""; tuning="no"])
+    AC_MSG_CHECKING([whether $CC accepts ${CFLAGS_TUNING}])
+    AC_COMPILE_IFELSE([ ],
+                      [tuning="yes"],
+                      [CFLAGS_TUNING=""; tuning="no"
+                       AS_IF([test "${with_tuning}"],
+                             [AC_MSG_ERROR([requested tuning not supported])])])
 
-        AC_MSG_RESULT([$tuning])
-        CFLAGS="${CFLAGS_save}"
-    ])
-fi
+    AC_MSG_RESULT([$tuning])
+    CFLAGS="${CFLAGS_save}"
+])
 
 dnl
 dnl  x86 accelerations